1. Field of the Invention
The present invention relates generally to fiber lasers, and in particular to wavelength selectable fiber lasers.
2. Technical Background
Optical fiber has become the transmission medium of choice for telecommunication networks operating at high data rates and over long distances. A single optical fiber has an optical data capacity measured in terabits per second. However, data rates in operational networks are usually limited by the electronics at either end of the fiber. Currently, electronic data rates are limited to about 10 gigabits per second (Gb/s), and the electronic equipment operating at such rates is expensive and difficult to control in a field environment. Further increases in data rates are possible, but not by dramatic amounts using conventional equipment. In fact, much of the advanced communications network being used today operates at no more than 1 Gb/s.
Nonetheless, the data capacity of fiber networks can be significantly increased, even in the case of already-installed fibers, by the use of wavelength-division multiplexing (WDM). In WDM, the data transmitter includes multiple lasers operating within a preferred transmission wavelength band, but at slightly different wavelengths within the band. Each wavelength represents a separate transmission channel with the channels spaced across the band. Separate data signals are impressed on the different channels either by electronically modulating the source laser itself or by optically modulating the laser's output. The different data channels are optically multiplexed into the transmitting end of the one fiber. At the receiving end, the different channels are optically demultiplexed into physically separated paths, and respective optical receivers detect the modulated signals. As a result, for example, a 16-channel WDM system operating with 10 Gb/s electronics has an effective total capacity of 160Gb/s. To maximize capacity over a single fiber, the WDM channels of current systems may typically be spaced on the order of 0.4 nm, which is equivalent to 50 GHz at these wavelengths. For long-distance transmission, one preferred wavelength band occurs from 1530 to 1580 nm, corresponding to the minimum attenuation or loss for silica fiber.
In a point-to-point WDM architecture for a fiber communication system, a bank of wavelength-differentiated transmitters on one end of a section of fiber is matched by a bank of corresponding receivers at the other end. However, modern networks have complicated topologies with many interconnected nodes at which it is desired to switch channels in different directions. An example of a relatively simple network is a dual-fiber ring with many nodes distributed around the ring. If the network path is broken or interrupted at any point, the data can be redirected in the opposite direction on the ring to avoid the interruption. Such a ring network can be made "self-healing." A characteristic of such networks is that a relatively large amount of traffic passes through a node compared to the traffic dropped or added at the node.
To implement a conventional point-to-point fiber design on a network requires a complete set of transmitters and receivers at each network node if the different WDM signals on the incoming fiber will be directed into different outgoing fibers. An electronic switch is also required at the node if the channels are being switched on a frequent basis. Optical receivers and transmitters are expensive, particularly in view of the signal conditioning and control functions required in a modern network such as those in the telecommunications industry, and in a realistic network it is desired to minimize their number.
All-optical networks have been proposed to reduce the cost and complexity of high-capacity, multi-node networks. In these networks, each node includes the capability of optical switching which allows the extraction or insertion of only some of the WDM wavelengths at the node without the need to demodulate to electronic signals those optical signals merely passing through the node. Such wavelength-selective extraction and insertion of an optical data channel may be performed by a wavelength add-drop multiplexer (WADM). If the WADM serves as the interface to an electronic switch or network, an optical transmitter at one or more of the WDM wavelengths is required in association with the WADM to insert a data signal into the fiber network. As a result of the optical switching, optical transmitters and receivers and associated electronics are required at a node only for signal wavelengths originating or terminating at that node.
In one example of wavelength assignment in an all-optical WDM network, a first wavelength is assigned for transmitting from a first terminal node to a second terminal node, a second wavelength from the first terminal node to a third terminal node, a third wavelength from the second terminal node to the third terminal node, and so on.
However, such a distributed network with the different nodes generating respective optical signals and inserting them onto the same network requires close registration of wavelengths between the nodes. For example, if one receiving node is receiving two closely spaced wavelengths originating from different transmitting nodes and if those signals drift differently with temperature or time, as is likely with two widely dispersed transmitters, the two signals may interfere and the receiver cannot adequately separate them.
The International Telephone Union (ITU) has defined a set of discrete WDM wavelengths in the 1530-1580 nm band with 100 GHz spacing in what is often referred to as the ITU grid. It is strongly desired that the source lasers conforming to the ITU grid have narrow linewidths of less than 1 MHz and exhibit a stable frequency over the full ambient temperature range between -40 and +85.degree. C. A maximum thermal frequency drift is required over this range of .+-.1 GHz (0.01 nm); less is desirable for close wavelength spacings. The source lasers should also have low relative intensity noise, which the ratio of signal to inherent noise.
Semiconductor lasers exhibit most of the characteristics required of source lasers, but the temperature dependence of their emission wavelength is very high. As a result, active temperature or wavelength control is required to maintain the output of semiconductor lasers within a narrow wavelength band despite wide excursions of the ambient temperature. Although close active control is available, it introduces undesirable complexity and cost for a fielded commercial system.
Fiber lasers have been developed which meet most of the requirements for a laser source in a fiber-based telecommunications system, including low relative intensity noise. Conventionally, a semiconductor diode laser or other intense light source optically pumps a fiber laser doped with specific ions to be optically active so that radiation in a fiber emission band is generated. A transmissive or reflective Bragg grating written into the fiber establishes the precise lasing wavelength within the fiber emission band. One of the frequencies of the ITU grid is selected for the Bragg grating. An advantage of writing the frequency into the silica fiber is that the silica has a small coefficient of thermal expansion (.about.5.times.10.sup.-7 /.degree. C.) and the resonant Bragg frequency changes with the same dependence. In particular, the center frequency of the Bragg grating in silica will normally vary by less than 10 GHz (0.1 nm) over a 100.degree. C. range and by much less than that for such a change in ambient temperature if the fiber laser is appropriately packaged.
However, fiber lasers have only recently been developed as sources for a WDM network. In the typical configuration, each WDM wavelength requires a separate fiber written with the proper WDM wavelength and having its own pump source.
In many telecommunications architectures, each transmitter may at any one time be transmitting at only one of the many wavelengths in the ITU wavelength grid or perhaps at a limited number of the ITU wavelengths. However, it is greatly desired that the node have a capability of changing its transmission wavelength between different wavelengths in the ITU grid. For example, as traffic patterns change, it is may be advantageous to reassign the wavelengths between the different nodes, and it is desired that the reassignment be accomplished without physically replacing the laser sources at all the affected nodes.
Furthermore, in the provisioning phase, it is desired that a separate part not be identified to each ITU frequency because the number of parts required to be inventoried in the warehouse or on the repair truck increases commensurately with the number of ITU frequencies being used. It is instead greatly preferred that the number of possible laser sources be reduced and that a single laser source be capable of emitting at any one of several of the ITU frequencies with the frequency selection of the laser source being achieved by a mechanical or preferably electrical adjustment in the field.
Multi-wavelength lasers are also needed for a wavelength-interchanging cross-connect (WIXC), which is an optical device linking two or more WDM networks. A WIXC has the capability of switching one or more selected signals from one WDM network to another with the carrier wavelengths being possibly changed between the two networks. The wavelength conversion conserves the number of wavelengths since only a limited amount of traffic is usually switched from one network to another. At the present time, the wavelength conversion requires demodulation to an electronic signal and a laser source emitting at the new wavelength and modulated by the electronic signal. For reasons based on both provisioning and wavelength reallocation, a multi-wavelength laser source is desired for a WIXC.
Furthermore, diode lasers are expensive and their cooling somewhat complex so that it is uneconomical to use more than are really necessary. On the other hand, diode lasers tend to experience short lifetimes in view of their usually high operating temperatures. It would be desirable to design a WDM system in which the number of laser diodes is reduced, but which also allows quick substitution for a failed laser. It is also desirable to increase the pumping power from diode lasers without the need to change the diode design.
Conventional fiber laser systems do not satisfy these different objectives.